Ceramic materials are becoming widely used in industry today. Ceramics because of their lower costs are being substituted for steel and other metal-based materials in the construction of a variety of devices and machining parts. For example, ceramics are being used as replacements for steel and other metal materials in the manufacture of car bodies, airplane parts, turbine blades, flow control valves, engine parts, metal cutting, forging dies, extrusion dies.
Sialon, an acronym for silicon aluminum oxynitride, is a particularly useful ceramic material which is generally prepared from a mixture of silicon nitride, alumina, aluminum nitride, and a sintering aid such as yttria. The problem with many ceramics, including sialon, is that treatments to increase their strength often deleteriously effect their fracture resistance and ceramics need to have superior characteristics in both areas, not just one, to be equivalent to the steel or other metal materials that they are seeking to replace. Moreover, valves and other engine components usually require grinding to specific shapes and tolerances and the grinding introduces surface damage that reduces the strength of the component. The strength reduction is greater when stressed in a direction perpendicular to the grinding direction which frequently occurs in complex parts such as valves. The machining damage often results in component failures in subsequent use.
Oxidation treatments have been used to partially heal machining stress crack damage with ceramics other than sialon, the problem with doing so for sialon is that the sialon is extremely susceptible to secondary recrystallization which can degrade the fracture toughness of the final body.
Accordingly, it is an object of the present invention to improve the strength properties of sialon without diminishing its fracture toughness.